<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8" />
		<title></title>
	</head>
	<body>
		
	</body>
	
	<script>
		
		var a = 10;

		function f(a){
			var a = 20;
			console.log(++a); //21
		}
		
		f(a);
		
		console.log(a); //10
		
		
		
		
		
	</script>
	
</html>